To date, the determination of the concentration of cyanide in solution has been largely limited to manual or semi-automated techniques due to the handling requirements of cyanide solutions. In particular, sample preparation prior to determination by silver nitrate titration has been largely limited to manual techniques due to the toxicity of HCN gas and the requirement to conduct sample preparation in a controlled environment. Nevertheless, some fully automated instruments have been developed for the analysis of free cyanide involving potentiometric determination.
“Free” cyanide analysis involves the determination of the amount of CN− and HCN species in a sample. In contrast, Weak Acid Dissociable (WAD) cyanide determination includes the amount of cyanide species liberated at moderate pH of 4.5, for example HCN(aq) and CN−. WAD cyanide determination accounts for the majority of Cu, Cd, Ni, Zn, Ag cyanide complexes and others compounds with similar low dissociation constants. That is, WAD cyanide determination is not limited to free cyanide only. As a result, and depending on the method used for determination, sample preparation for the determination of WAD cyanide can be significantly more complicated than that for free cyanide determination.
WAD cyanide analysis samples should be free from interferences due to the presence of high concentrations of more stable cyanide complexes or other cyanide forms. If not, the interference must be quantified and allowed for in the result. Thus, the automation of WAD cyanide analysis is less common.
It is not necessary to account for free cyanide in a WAD cyanide analysis, because WAD cyanide encompasses all free and weakly bound complexes. In fact, in the majority of the known techniques the measured WAD cyanide is derived from the amount or concentration of free cyanide in the sample.
Some automated WAD analysers have been developed using amperometric determination or colorimetric determination techniques. However, these methods have been known to exhibit inaccuracies of up to 25%. A more preferable determination method for the analysis of WAD cyanide is silver nitrate potentiometric titration due to the high level of accuracy that can be achieved. This is also the preferred method recommended by the International Cyanide Management Code developed for the gold mining industry. Unfortunately, this method of determination involves complicated sample pre-treatment, including distillation, which is not required for the colorimetric or amperometric techniques. Some other known automated on-line WAD cyanide analysers use techniques including the picric acid method of determination and ligand exchange.
A known laboratory method for measuring complex cyanide solutions involves distilling a 500 ml sample of a cyanide solution in a flask fitted with a thistle tube. During distillation HCN gas is liberated from the cyanide solution. Air is drawn through the thistle tube by suction, which causes the vapour and gasses from the flask to be drawn through a condenser attached to the flask. The gasses are then drawn through a caustic solution in an absorption tube where HCN gas liberated from the distillation step is then converted back to CN−.
Using this method, any air and/or remaining gas drawn through the caustic solution is simply released to atmosphere. As such, the gas flow rate has to be very slow to ensure as much of the HCN gas as possible is absorbed by the caustic solution to minimise losses of HCN. The process takes approximately 1.5 hours, after which the absorption tube is removed and the sample taken away for analysis. Great care must also be taken to ensure all of the solution is removed from the tube prior to measurement to prevent losses and ensure an accurate quantitative measurement.
The flow rate must also be controlled for health and safety reasons. A slow flow rate may be required to ensure that all toxic HCN gas is absorbed in order to prevent the release of HCN gas.
Furthermore, cyanide samples tend to degrade over time and, although preservation techniques are available, it is preferable that any analysis be completed as soon as possible.
It is desired to address or ameliorate one or more disadvantages or shortcomings associated with existing chemical analysis apparatus and methods, and/or to at least provide a useful alternative thereto. The present invention seeks to reduce analysis times, losses of solution leading to inaccuracies and the requirement for manual handling of the sample before or during the analysis.